Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
A brushless DC motor (BLDC) usually includes windings (or a stator) that are stationary and magnets (or a rotor) rotate relative to the windings. Example BLDC configurations place the rotor internal to the stator such that the external motor housing is stationary. This allows the housing to be attached to a fixed chassis with no external moving parts.
An outrunner brushless DC motor configuration includes the rotor external to the stator such that the external motor housing is spinning. Thus, for this type of motor, an outer shell of the motor spins around the internal windings. The stator is typically connected to a faceplate at a front of the motor and a back of the motor spins. This faceplate can be attached to the fixed chassis. Outrunner BLDC motors are common and can be found in CD-ROM computer drives, radio-controlled (RC) airplanes, and electric bicycles, for example.
In many cases, it is desired to attach a motor angle sensor, such as an encoder, to the motor rotor to provide advanced control of the motor. With a standard BLDC motor, this can be accomplished by attaching the encoder to a back of the motor such that a head of the encoder is fixed to the motor housing and a wheel of the encoder is attached to a back of the rotating motor shaft. The head and wheel of the encoder can be registered directly to local features on the motor housing to ensure precise concentricity and angular alignment. Poor alignment can cause encoder errors.
Such a mounting arrangement is generally not possible for outrunner motors as the motor housing is rotating. Instead, sometimes the head of the encoder is mounted to the chassis while the wheel is mounted to the motor housing, which can create a larger tolerance loop than in the standard configuration. Precise alignment of the head and wheel are dependent on chassis tolerances, and therefore, encoder performance can be more susceptible to errors. This may also increase complexity of the chassis structure and a size of the overall packaging.